The invention relates to caustic-based metal batteries and methods for their operation, and particularly aluminum anode batteries having recirculating caustic electrolyte containing a particulate seeding agent.
Caustic-based metal batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell. During cell operation oxygen is reduced within the cathode while metal of the anode is oxidized, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added oxygen-reduction catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element A variety of anode metals have been used or proposed; among them, zinc, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal/air battery using a variety of electrolytes.
Other caustic-based metal batteries may utilize a cathode comprising a nickel surface impregnated with a catalyst. This cathode does not have a surface exposed to air or oxygen. Instead, the liquid electrolyte contains hydrogen peroxide in solution which is reduced by the cathode while the metal of the anode is oxidized, thereby providing a useable electric current flow.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air or oxygen, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface.
The aqueous electrolyte for caustic-based aluminum batteries typically consists of a highly alkaline solution. The highly alkaline electrolyte usually consists of NaOH or KOH solution.
In alkaline electrolyte, the cell discharge reaction may be written: EQU 4Al+3O.sub.3 +6H.sub.2 O+4KOH.fwdarw.4Al(OH).sub.4.sup.- +K.sup.+ (liquid solution),
followed, after the dissolved potassium (or sodium) aluminate exceeds saturation level, by: EQU 4Al(OH).sub.4.sup.- +4K.sup.+ .fwdarw.4Al(OH).sub.3 (solid)+4KOH
In addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows: EQU 2Al+6H.sub.2 O .fwdarw.2Al(OH).sub.3 +3H.sub.2 (gas)
There is a need for a caustic-based aluminum battery which can be used as an emergency power source at locations where electric supply has been temporarily disrupted. Such a battery must have a high energy capacity and be capable of running for a long period of time under high load, e.g. deliver in excess of 500 watts with an energy density in excess of 365 Wh/kg. During discharge of a battery containing aluminum anodes and caustic electrolyte, the concentration of dissolved aluminum in the electrolyte continues to build up until a limiting level of super- saturation is reached such that no more aluminum from the anode can enter into solution. At this point a film or scale of aluminum hydroxide forms on the anode surface causing passivation of the anode and collapse of the battery voltage.
The solubility of aluminum hydroxide increases with temperature and with caustic concentration. In metal-air batteries, caustic concentrations are chosen to maximize electrical conductivity and are typically in the range of 4-5 molar. At this caustic level the aluminum solubility at the prevailing battery temperature of 55.degree.-75.degree. C., corresponds to a molar ratio of dissolved Al to KOH or NaOH of roughly 0.40. Aluminum may continue to dissolve above this ratio into the supersaturated zone and even attain a ratio as high as 0.80. In the supersaturation zone the solution is in a metastable state and has a natural tendency to reduce its dissolved aluminum concentration by precipitating out solid aluminum-oxide trihydrate or bayerite according to the following equation: EQU Al(OH).sub.4.sup.- .fwdarw.Al(OH).sub.3 .dwnarw.+OH.sup.- EQU or, EQU 2Al(OH.sub.4.sup.- .fwdarw.Al.sub.2 O.sub.3.3H.sub.2 O.dwnarw.+2OH.sup.-
The metastable state possesses great stability, unless a seeding agent is present to induce precipitation, and the Al/XOH ratio can go as high as 0.75-0.80, where anode passivation occurs. High current density and low electrolyte turbulence enhance the tendency to passivate.
Various techniques have been tried to avoid passivation at medium to high current densities and one technique has been to use a very large volume of electrolyte. However, this greatly increases the battery size and weight, thus reducing its energy density and market attractiveness. Another way of extending discharge time prior to passivation has been to use higher caustic concentrations, but this has the effect of reducing electrolyte conductivity and hence battery voltage. Yet another way has been to add organic stabilizers to the electrolyte to improve the meta-stability, or to use NaOH/KOH mixtures to achieve the same effect, but these methods achieve only a relatively small extension of the battery capacity.
Even if the anode does not passivate, there is a further problem with the occurence of very high supersaturation levels corresponding to Al/XOH ratios above 0.70 and that is the uncontrolled nucleation of a vast number of new particles of solid aluminum hydroxide, probably close to the anode surface. These have the effect of increasing the viscosity of the electrolyte to that of a thick soup and thereby clogging the cell and causing battery failure.
There remains a need for a seeding agent to induce hydrargillite or bayerite precipitation which is insoluble in caustic electrolyte, while having a fineness of particle size allowing it to remain suspended in the electrolyte with little or no mechanical stirring.